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| [GMCnet] IR Reflector from Home Depot [message #90621] |
Thu, 01 July 2010 07:34  |
Ken Coit
Messages: 151
Registered: November 2005
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I was cruising the aisles at Home Depot yesterday and came away with 48 sq.
ft. of Enerflex for about $0.50 a sq. ft. I don't know if it will stand up
to buffetting on the tanks unprotected, but I'll bet it is good enough for
the engine hatch. windows, and other protected environments like my
skylights. It is said to be as much as 96% IR reflective. It rolls pretty
tight and has a mesh in it that keeps it from crumbling like aluminum foil.
Has anyone used Enerflex or a similar product? How did you apply it?
--
Ken Coit, ND7N
Raleigh, NC
Parfait Royale
1978 Royale Rear Bath, 403, 3.07
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| Re: [GMCnet] IR Reflector from Home Depot [message #90658 is a reply to message #90621] |
Thu, 01 July 2010 10:17  |
Rick Denney
Messages: 430
Registered: January 2004
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| Ken Coit wrote on Thu, 01 July 2010 08:34 |
Has anyone used Enerflex or a similar product? How did you apply it?
|
Remember that heat can be transferred in one of several ways. One is by radiation, including infrared. A shiny barrier will block a good bit of this radiation. Inside an engine compartment, it will keep radiative heat from coming up through the floor, but it will increase the temperature in the compartment.
Another type of heat transfer is conductive. Heat conduction is just like electricity conduction. Heat transfers through the solid material connecting the hot thing with the less-hot thing. This is what heats up a carburetor sitting atop a very hot manifold. The solution is to install insulation in the conduction path, which is one reason we use a thick fiber gasket between the carb and the manifold.
The third type is convection, where the hot thing heats the air around it, which then moves to the cooler thing and gives up its heat. The only way to stop convective heat transfer is to 1.) stop the air from moving, and 2.) eliminate the open air path between the hot thing and the less-hot thing.
So, when we put a shiny reflector in the engine compartment, we turn radiation around. But it doesn't do as much for conduction or convection, especially if the shiny material is tight up against the thing we are trying to keep cool. Many of these radiant barriers are bonded with insulation, and the insulation blocks against conduction and convection.
But conduction and convection are insidious. You can't eliminate them, you can only slow them down. If I put insulation over, say, a fuel tank, my objective is slow down heat transfer to the point where I can either conduct or convect the heat to some other destination before it is expended raising the temperature of the fuel. Tight insulation provides this slowdown, but it also prevents the air flow from cooler air sources that might carry the heat away. So, I don't think I want tight insulation around the tanks, unless I have another means of cooling the fuel within the tanks. The radiant shield with the air gap breaks the conduction barrier (with an air gap) and also improves convection. It's primarily effective against radiant heat. But what makes it work is the air flow behind the shield. We hope that the air flow behind the shield will take heat away at the reduce rate at which it is getting through the shield.
Insulation on the floor is another matter. The other side of the floor is being actively cooled, so anything we do to slow down heat transfer keeps the heat from getting the upper hand on the cooling.
These principles are why I'm not particularly impressed by demonstrations of a flame on one side of a piece of ceramic paper and the demonstrator's hand on the other side. I would be more impressed if one left the flame on that spot for, say, five minutes, and still found the other side to be cool. A thin piece of ceramic paper will slow down heat transfer for a bit, but mostly it's a flame barrier.
And this has raise an issue in my mind. I have a reflective radiant barrier on the underside of my hatch, and now I'm wondering how much that is increasing the heat above the engine. I'm now thinking that a black insulation layer with a ventilated air space behind it will do much more to pull heat away from the top of the engine.
Also, I'm going back to the remote cool air entry for the carb.
Rick "rethinking engine compartment heat" Denney
'73 Glacier 230 "Jaws"
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| Re: [GMCnet] IR Reflector from Home Depot [message #90666 is a reply to message #90658] |
Thu, 01 July 2010 11:05 |
Ken Coit
Messages: 151
Registered: November 2005
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Thanks Rick. I find nothing to disagree with in your entire piece. Well, I
might not encourage heat to reach the wooden floor even if it does warm the
engine compartment. I'll run some experiments with theremocouples, metal
plates, insulation, and reflectors to see what the sun does to each.
Radiation is about all we can fight at the fuel tanks without a cooling
system or fans that blow on them when they get too warm at standstill.
Maybe a misting swamp cooler?
Later,
Ken
On Thu, Jul 1, 2010 at 11:17 AM, Rick Denney <rick@rickdenney.com> wrote:
>
>
> Ken Coit wrote on Thu, 01 July 2010 08:34
> > Has anyone used Enerflex or a similar product? How did you apply it?
>
>
> Remember that heat can be transferred in one of several ways. One is by
> radiation, including infrared. A shiny barrier will block a good bit of this
> radiation. Inside an engine compartment, it will keep radiative heat from
> coming up through the floor, but it will increase the temperature in the
> compartment.
>
> Another type of heat transfer is conductive. Heat conduction is just like
> electricity conduction. Heat transfers through the solid material connecting
> the hot thing with the less-hot thing. This is what heats up a carburetor
> sitting atop a very hot manifold. The solution is to install insulation in
> the conduction path, which is one reason we use a thick fiber gasket between
> the carb and the manifold.
>
> The third type is convection, where the hot thing heats the air around it,
> which then moves to the cooler thing and gives up its heat. The only way to
> stop convective heat transfer is to 1.) stop the air from moving, and 2.)
> eliminate the open air path between the hot thing and the less-hot thing.
>
> So, when we put a shiny reflector in the engine compartment, we turn
> radiation around. But it doesn't do as much for conduction or convection,
> especially if the shiny material is tight up against the thing we are trying
> to keep cool. Many of these radiant barriers are bonded with insulation, and
> the insulation blocks against conduction and convection.
>
> But conduction and convection are insidious. You can't eliminate them, you
> can only slow them down. If I put insulation over, say, a fuel tank, my
> objective is slow down heat transfer to the point where I can either conduct
> or convect the heat to some other destination before it is expended raising
> the temperature of the fuel. Tight insulation provides this slowdown, but it
> also prevents the air flow from cooler air sources that might carry the heat
> away. So, I don't think I want tight insulation around the tanks, unless I
> have another means of cooling the fuel within the tanks. The radiant shield
> with the air gap breaks the conduction barrier (with an air gap) and also
> improves convection. It's primarily effective against radiant heat. But what
> makes it work is the air flow behind the shield. We hope that the air flow
> behind the shield will take heat away at the reduce rate at which it is
> getting through the shield.
>
> Insulation on the floor is another matter. The other side of the floor is
> being actively cooled, so anything we do to slow down heat transfer keeps
> the heat from getting the upper hand on the cooling.
>
> These principles are why I'm not particularly impressed by demonstrations
> of a flame on one side of a piece of ceramic paper and the demonstrator's
> hand on the other side. I would be more impressed if one left the flame on
> that spot for, say, five minutes, and still found the other side to be cool.
> A thin piece of ceramic paper will slow down heat transfer for a bit, but
> mostly it's a flame barrier.
>
> And this has raise an issue in my mind. I have a reflective radiant barrier
> on the underside of my hatch, and now I'm wondering how much that is
> increasing the heat above the engine. I'm now thinking that a black
> insulation layer with a ventilated air space behind it will do much more to
> pull heat away from the top of the engine.
>
> Also, I'm going back to the remote cool air entry for the carb.
>
> Rick "rethinking engine compartment heat" Denney
> --
> '73 Glacier 230 "Jaws"
> _______________________________________________
> GMCnet mailing list
> List Information and Subscription Options:
> http://temp.gmcnet.org/cgi-bin/mailman/listinfo/gmclist
>
--
Ken Coit, ND7N
Raleigh, NC
Parfait Royale
1978 Royale Rear Bath, 403, 3.07
_______________________________________________
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